Electrical power converter systems are widely used to convert fixed frequency alternating current (AC) power to variable frequency, variable amplitude power. In general, such systems include a rectifier for converting AC power to direct current (DC) power and an inverter for converting the DC power to variable frequency, variable amplitude AC power. The output AC power from the inverter may be pulse width modulated (PWM) or otherwise controlled by the inverter to establish a desired output power level. In such systems, a DC link transfers power from the rectifier to the inverter. The DC link may be merely an inductive coupler for transiently isolating the rectifier from the inverter.
Inverters may be of the current source type or the voltage source type. The voltage source inverter, such as the PWM inverter, is well known in the art and is used in motor drive applications as well as in voltage regulated frequency changer applications. The present invention is generally directed to voltage source inverters and is described in that context. However, both types of inverters employ a plurality of controlled semiconductor switching devices which are subject to switching losses and potential thermal failures for which the present invention provides a means of reducing.
One source of switching losses in electrical switching converters is attributable to the switching frequency of the converters. It is desirable to reduce the size and weight of electrical switching converters and one method of doing so is to increase the converter switching frequency. By increasing the switching frequency, the passive components such as inductors and capacitors operate at a higher frequency and hence can be made smaller, resulting in a converter of smaller size, lower weight, and faster response time. However, the higher frequency switching aggravates switching device losses and degrades efficiency of the converter.
The voltage source inverter utilizes semiconductor switches which must withstand both high voltage and high current during switching transients. Consequently, these switches suffer from high switching losses. Some attempts to reduce switching losses have involved employing passive snubber circuits, each connected to a respective one of the switching devices. This requires use of many added components which may themselves be lossy. Another proposed method employs a resonant DC link, but this method requires that the link switches withstand two times the DC input voltage and carry high voltage and current since the energy processed flows through these switches.